In power generation, at a specified output, an increase of the rotary speed of a turbine is associated with a decrease in size and costs. Efficiency, too, can be improved. So far, power generation turbines up to 70 MW are connected to generators by way of gearing arrangements, so as to allow operation at higher turbine rotary speeds. As the output increases, the use of gearing arrangements becomes increasingly difficult due to reliability reasons. In such cases, the turbine is operated at synchronous speed.
The use of static frequency converters (power electronics) represents an alternative providing many advantages such as reduced costs of the generator in agreement with a constant product of volume and rotational speed, standardised generators for both 50 and 60 Hz, adjustable speed which allows restoration of the partial-load efficiency of the turbine, substantial reduction in noise, clean (oil-free) cooling, etc.
Both in the case of power generation and in the case of drives, a reduction in losses of the static frequency converters would bring about substantial cost savings. A reduction of the losses would above all have a bearing on investment costs because cooling accounts for a substantial part of the total costs of the converter.
Static frequency converters exist both with indirect AC/DC/AC conversion and with direct AC/AC conversion.
The indirect conversion (AC/DC/AC) is caused by generating a direct current or a direct voltage from the three-phase source (mains in the case of motors; generator in the case of power generation). Subsequently, the direct current or the direct voltage is converted back to an alternating current by means of an inverter. An inductance (current source converter) or a capacitor bank (voltage source converter) are switched into the intermediate circuit so as to reduce the ripple component of the current or the spikes.
Today's converters make use of thyristors. If natural commutation of the thyristors is possible, the losses in the converter are reduced. Voltage source converters use GTOs with their inherent high switching losses, as well as IGBTs or IGCTs. The power capability of the individual components is less than that of thyristors, consequently, a larger number of components are required for a specified voltage and a specified current. Voltage source converters can benefit from the use of pulse-width modulation techniques, which improve the shape of the current curves and reduce the harmonics. The higher the switching frequencies the better, except with regard to losses and dielectric fatigue. The current can largely be produced sine-shaped so that a derating of power of the electrical machine is avoided.
Direct conversion (AC/AC) is for example possible by means of a so-called cyclo-converter. Direct conversion provides significant advantages from the point of view of the electrical machine, because the current is more or less sine-shaped rather than chopped direct current. It reduces the losses that occur additionally in the electrical machine and it also prevents pulsating torques.
However, the use of a 3-phase cyclo-converter limits the achievable frequency range to 0-⅓ of the input frequency. A 3-phase cyclo-converter is made of 3 single phase cyclo-converters, each processing ⅓ of the power in balanced operation. Exceeding the ⅓ limit in frequency ratio results in a strongly unbalanced operation. In this case each single phase cyclo-converter should be designed for more than ⅓ of the full power. The overdimensioning can be up to a factor of 3 in power rating.
Another possibility of direct conversion is provided by a so-called matrix converter in which each phase of a multi-phase source (generator or mains) is connected or connectable with each phase of a multi-phase load (mains, passive load, motors, etc.) by a bi-directional switch. The switches consist of an adequate number of thyristors to withstand the differential voltage between the phases, and the phase currents, and to allow current reversal. They can be regarded as truly bi-directional components with the options of jointly using additional wiring such as snubbers or the gate unit power supplies for the drive pulses for the antiparallel components.
The switches are arranged in a (m×n)-matrix at m phases of the source and n phases of the load. This provides the option of establishing any desired connections between the input phases and the output phases; however at the same time it has the disadvantage in that certain switching states of the matrix must not be allowed since otherwise for example a short circuit would result. Furthermore it is desirable to carry out commutation from one phase to another phase such that the lowest possible switching losses result.
U.S. Pat. No. 5,594,636 describes a matrix converter and a process for its operation in which commutation between the phases is partly carried out as a natural commutation, with a forced commutation where natural commutation is not possible. Although with this type of selection, switching losses are reduced due to natural commutation, those switching losses that arise from forced commutation still remain. Furthermore, the possible forced commutation necessitates the use, in all positions on the matrix, of components that can be switched off. This considerably increases the switching expenditure.
However, it is possible to operate a matrix converter in a way that only natural commutations are being used. This can be achieved by only allowing the switching over from a selected connected phase of the generator to a selected not connected phase of the generator only if certain conditions are met. Such a matrix converter as well as a mode of its operation has been disclosed in DE 10051222 A1 as well as in the corresponding European application EP-A-1199794. While being of high efficiency and versatility, the concept of a matrix converter and its mode of operation generally suffer for certain applications, from weaknesses with respect to harmonic distortion and with respect to possible frequency ratios.